Color Liquid Crystal Display Device And Gamma Correction Method For The Same

ABSTRACT

A color liquid crystal display device configured to employ a pixel division method in which each pixel of a displayed image is configured with sub-pixels obtained by spatial division of one pixel in a division ratio may include: pixel formation portions provided correspondingly to respective pixels of the image, each portion configured to form a pixel of primary colors with the sub-pixels; a drive circuit configured to provide each portion with applied voltages respectively corresponding to the sub-pixels composing the pixel to be formed by that portion, based on a gradation value indicated by an input signal provided as a video signal representing the image; and a gamma correction part configured to correct a relationship between the gradation value indicated by the input signal and a luminance value of the pixel to be formed by that portion according to the gradation value independently for each of the primary colors.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a divisional of U.S. patent application Ser. No.12/083,011, filed on Apr. 3, 2008, in the U.S. Patent and TrademarkOffice, the entire contents of which are incorporated herein byreference. U.S. patent application Ser. No. 12/083,011 is a nationalstage entry from International Application No. PCT/JP2006/310853, filedon May 31, 2006 (and amended on Feb. 16, 2007), in the Receiving Officeof the Japan Patent Office, the entire contents of which are alsoincorporated herein by reference. International Application No.PCT/JP2006/310853 claims priority from Japanese Patent Application No.2005-316676, filed on Oct. 31, 2005, in the Japan Patent Office (JPO),the entire contents of which are additionally incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to a color liquid crystal display deviceemploying a pixel division method, in which each pixel of a displayedimage is composed of a predetermined number of two or more sub-pixelsobtained by spatial or temporal division of one pixel, and morespecifically to an improvement of color reproducibility in such a colorliquid crystal display device.

BACKGROUND ART

Usually in a display device, for the purpose of a good reproduction ofan image represented by a video signal provided from outside as an inputsignal, gradation or the like indicated by the input signal is correctedfor adjusting a relationship between a gradation value indicated by theinput signal and a luminance value of an image actually displayed. Suchcorrection is called “gamma correction”.

A liquid crystal display device displays an image represented by aninput signal by controlling an applied voltage of liquid crystalaccording to the input signal and thereby changing light transmittanceof the liquid crystal. In such a liquid crystal display device, thegamma correction is also carried out by correcting the gradation valueor the like indicated by the input signal according to a relationshipbetween the applied voltage and the transmittance of the liquid crystal(hereinafter, referred to as “VT characteristics”).

Meanwhile, the liquid crystal display device controls the transmittanceby applying a voltage across a liquid crystal layer sandwiched between apair of polarizer plates and thereby changing a phase difference(retardation) of the liquid crystal layer. Recently, a verticalalignment (VA) mode of the liquid crystal is used for an application toa television (TV) and a monitor, which is a normally black mode showinga black image without the applied voltage and provides a high qualityblack image and a high contrast. In this VA mode, the retardation of theliquid crystal has a wavelength dependence. Therefore, in a color liquidcrystal display device which displays a color image using three kinds ofpixels, R (red), G (green), and B (blue), the VT characteristics areslightly different among the three kinds of pixels.

Accordingly, there has been conventionally proposed a liquid crystaldisplay device which carries out the gamma correction independently foreach R, G, and B for obtaining a good color reproducibility in adisplayed image (hereinafter, such a gamma correction carried outindependently for each R, G, and B is referred to as “RGB independentgamma correction”, or simply “independent gamma correction”). Forexample, Japanese Unexamined Patent Application Publication No.2002-258813 (patent reference 1) discloses a color liquid crystaldisplay device which determines γ-curves of R, G, and B individually bygenerating gradation voltages independently for each R, G, and B(carries out the independent gamma correction). Also, Japanese

Unexamined Patent Application Publication No. 2001-222264 (patentreference 2) discloses a liquid crystal display device including astorage means for storing gamma correction data for R, G, and Bgenerated on the basis of each luminance characteristics of an R pixel,G pixel, and B pixel arranged in a matrix on a liquid crystal panel, anda gamma correction means for individually correcting an R signal, Gsignal, and B signal composing a video signal to be supplied to the Rpixel, G pixel, and B pixel, respectively, on the basis of the gammacorrection data for R, G, and B (carrying out the independent gammacorrection).

-   Patent reference 1: Japanese Unexamined Patent Application    Publication No. 2002-258813-   Patent reference 2: Japanese Unexamined Patent Application    Publication No. 2001-222264-   Patent reference 3: Japanese Unexamined Patent Application    Publication No. 2004-78157-   Patent reference 4: Japanese Unexamined Patent Application    Publication No. 2004-62146-   Patent reference 5: Japanese Unexamined Patent Application    Publication No. 2005-173573

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

In a liquid crystal display device using a VA mode, γ-characteristicsare different between a case where a display screen is viewed from thefront thereof (in a front view) and a case where the display screen isviewed with an angle (in an oblique view). Therefore, the transmittanceof the liquid crystal in the oblique view becomes higher than that inthe front view and display on the screen appears as floating in white(white floating). Various methods have been proposed for improving suchwhite floating in the oblique view (more generally, for improving aviewing angle dependence of the γ-characteristics).

For example, Japanese Unexamined Patent Application Publication No.2004-78157 (patent reference 3) and Japanese Unexamined PatentApplication Publication No. 2004-62146 (patent reference 4) discloseliquid crystal display devices employing a pixel division method whichcan improve the viewing angle dependence of the γ-characteristicsrepresenting a relationship between the gradation value and the displayluminance value. Each of these liquid crystal display devices employingthe pixel division method has a plurality of pixels arranged in amatrix, each having a liquid crystal layer and a plurality of electrodesfor applying voltages on the liquid crystal layer. Each of the pluralityof pixels has a first sub-pixel and a second sub-pixel where voltagesdifferent from each other can be applied on the liquid crystal layer,respectively. In such a configuration, a luminance value (ortransmittance value) of each pixel is provided on the basis of luminancevalues (or transmittance values) different from each other in the firstsub-pixel and the second sub-pixel, respectively. Providing a differenceof the luminance (or transmittance) between sub-pixels in one pixel inthis manner improves the viewing angle dependence of theγ-characteristics.

Here, while each pixel is divided spatially into the plurality ofsub-pixels (the first sub-pixel and the second sub-pixel) in theseliquid crystal display devices, instead each pixel may be configured tobe divided temporally into a plurality of sub-pixels, that is, may beconfigured such that one frame period is divided into a plurality ofsub-frames, a luminance difference is provided among the plurality ofsub-frame periods, and an average luminance value of the plurality ofsub-frame periods becomes a luminance value of each pixel (refer toe.g., Japanese Unexamined Patent Application Publication No. 2005-173573(patent reference 5)). A temporal pixel division method as in the lattercase also can improve the viewing angle dependence of theγ-characteristics.

Also in the liquid crystal display device employing the pixel divisionmethod (spatial or temporal) as described above, the independent gammacorrection is carried out and a gradation dependence of chromaticity issuppressed to obtain a good color reproducibility in displaying animage. In the pixel division method, however, even if the chromaticityis adjusted not to change depending on the gradation, that is, goodchromaticity characteristics maintaining a color balance are obtained inan almost whole gradation range when a screen of the liquid crystaldisplay device is viewed from a front direction, the chromaticity stillchanges depending on gradation values in a certain range of halftonewhen the screen is viewed from a oblique direction, as shown in FIG. 10.As a result, even in a case where the gradation value is changed suchthat each of the three primary colors, R, G, and B is mutually the same,that is, even in a case where the gradation value is changed such thatthe screen exhibits an achromatic color when viewed from the frontdirection as shown in (A) of FIG. 17, there is caused a phenomenon thatthe screen is tinged with yellow in a certain range of the halftone whenthe screen is viewed in the oblique direction, as shown in (B) of FIG.17. This means that the good color reproducibility can not be obtainedwhen the screen is viewed in the oblique direction.

Accordingly, an object of the present invention is to provide a colorliquid crystal display device which can display an image having a highcolor reproducibility when viewed from an oblique direction as well asfrom a front direction of a screen thereof, while improving the viewingangle dependence of the γ-characteristics by employing the pixeldivision method.

Measures for Solving the Problems

In a first aspect of the present invention, there is provided a colorliquid crystal display device, employing a pixel division method inwhich each pixel of an image displayed in a predetermined screen isconfigured with a predetermined number of two or more sub-pixelsobtained by spatial or temporal division of one pixel in a predetermineddivision ratio, the device comprising:

a plurality of pixel formation portions provided correspondingly torespective pixels of the image, each of the portions forming a pixel ofany of primary colors for color display with the predetermined number ofsub-pixels;

a drive circuit for providing each of the pixel formation portions withapplied voltages respectively corresponding to the sub-pixels composingthe pixel to be formed by that pixel formation portion, based on agradation value indicated by an input signal provided from outside as avideo signal representing the image; and

a gamma correction part for correcting a relationship between agradation value indicated by the input signal and a luminance value ofthe pixel formed by the pixel formation portion according to thegradation value independently for each of the primary colors for colordisplay,

wherein each of the pixel formation portions forms the pixel bydisplaying the predetermined number of sub-pixels with luminance valuesdifferent from one another based on the applied voltages, and

wherein the gamma correction part corrects the relationship such thatgradation dependence of chromaticity is suppressed when the screen isviewed from a front thereof, and also corrects the relationship in thevicinity of a predetermined gradation value, which is determined by thedivision ratio in the one pixel and differences in the applied voltageamong the predetermined number of sub-pixels, such that the gradationdependence of the chromaticity is suppressed when the screen is viewedfrom a predetermined oblique direction.

In a second aspect of the present invention, there is provided the colorliquid crystal display device according to the first aspect of thepresent invention, wherein the gamma correction part corrects thechromaticity when viewed from the front to be shifted from a statemaintaining a color balance toward blue in the vicinity of thepredetermined gradation value such that the gradation dependence of thechromaticity is suppressed when viewed from the oblique direction.

In a third aspect of the present invention, there is provided the colorliquid crystal display device according to the first aspect of thepresent invention, wherein the gamma correction part corrects therelationship such that a curve representing the gradation dependence ofthe chromaticity when viewed from the front becomes approximately flatin a range except for the vicinity of the predetermined gradation value.

In a fourth aspect of the present invention, there is provided the colorliquid crystal display device according to the first aspect of thepresent invention, wherein the gamma correction part corrects therelationship such that a curve representing the gradation dependence ofthe chromaticity when viewed from the front changes approximatelymonotonically with respect to the gradation value.

In a fifth aspect of the present invention, there is provided the colorliquid crystal display device according to the first aspect of thepresent invention, wherein the gamma correction part includes acorrection table associating a gradation value before correction with agradation value after correction for each of the primary colors forcolor display in order to correct the relationship, and outputs thegradation value after correction associated with the gradation valueindicated by the input signal referring to the correction table, and

wherein the drive circuit provides each of the pixel formation portionswith the applied voltage based on the gradation value after correction.

In a sixth aspect of the present invention, there is provided the colorliquid crystal display device according to the first aspect of thepresent invention, further including

a common electrode provided commonly at the plurality of pixel formationportions,

each of the pixel formation portions including:

a first and a second sub-pixel electrodes disposed facing the commonelectrode so as to sandwich a liquid crystal layer in between;

a first auxiliary electrode disposed so as to form a first auxiliarycapacitance between the first sub-pixel electrode and the same; and

a second auxiliary electrode disposed so as to form a second auxiliarycapacitance between the second sub-pixel electrode and the same, and

the drive circuit including:

a pixel electrode drive circuit for providing a voltage according to theinput signal to the first and second sub-pixel electrodes with thecommon electrode as a reference; and

an auxiliary electrode drive circuit for applying voltages which aredifferent from each other and changes in a predetermined period and apredetermined amplitude, to the first and second auxiliary electrodes,

wherein the predetermined gradation value is determined by an area ratioof the first sub-pixel electrode to the second sub-pixel electrode and adifference in the applied voltage between the first auxiliary electrodeand the second auxiliary electrode.

In a seventh aspect of the present invention, there is provided a gammacorrection method for correcting a relationship between a gradationvalue indicated by an input signal provided from outside as a videosignal representing an image and a luminance value of a pixel formedaccording to the gradation value, in a color liquid crystal displaydevice employing a pixel division method in which each pixel of theimage displayed on a predetermined screen is composed of a predeterminednumber of two or more sub-pixels obtained by spatial or temporaldivision of one pixel in a predetermined division ratio, the methodincluding

a correction step of correcting the relationship independently for eachof primary colors for color display,

wherein in the correction step, the relationship is corrected such thatgradation dependence of chromaticity is suppressed when the screen isviewed from a front thereof, and the relationship in the vicinity of apredetermined gradation value is also corrected such that the gradationdependence of the chromaticity is suppressed when the screen is viewedfrom a predetermined oblique direction, the predetermined gradationvalue being determined by the division ratio in the one pixel anddifferences in a voltage applied to liquid crystal among thepredetermined number of sub-pixels.

In an eighth aspect of the present invention, there is provided thegamma correction method according to the seventh aspect of the presentinvention, wherein in the correction step, the chromaticity when viewedfrom the front is corrected to shift from a state maintaining a colorbalance toward blue in the vicinity of the predetermined gradation valuesuch that the gradation dependence of the chromaticity is suppressedwhen viewed from the oblique direction.

In a ninth aspect of the present invention, there is provided the gammacorrection method according to the seventh aspect of the presentinvention, wherein in the correction step, the relationship is correctedsuch that a curve representing the gradation dependence of thechromaticity when viewed from the front becomes approximately flat in arange except for the vicinity of the predetermined gradation value.

In a tenth aspect of the present invention, there is provided the gammacorrection method according to the seventh aspect of the presentinvention, wherein in the correction step, the relationship is correctedsuch that a curve representing the gradation dependence of thechromaticity when viewed from the front changes approximatelymonotonically with respect to the gradation value.

Advantages of the Invention

According to the first aspect of the present invention, an independentgamma correction is carried out such that the gradation dependence ofthe chromaticity is suppressed when the screen is viewed from the frontof the screen (in the front view), and also the independent gammacorrection is carried out in the vicinity of the predetermined gradationvalue, which is determined by the division ratio in one pixel anddifferences in the applied voltage among the predetermined number ofsub-pixels in one pixel, such that the gradation dependence of thechromaticity is suppressed when the screen is viewed from apredetermined oblique direction (in the oblique view). Such anindependent gamma correction suppresses color imbalance in the range ofthe halftone observed in the conventional color liquid crystal displaydevice employing the pixel division method to such an extent thatmatters little for a human visual sense even in the oblique view, andthere is obtained a situation in which the color balance is maintainedsubstantially for the almost whole gradation range in the oblique viewas well as in the front view (to the extent that matters little for ahuman visual sense). As a result, it is possible to display an imagehaving a high color reproducibility when viewed from an obliquedirection as well as when viewed from the front of the screen, whileimproving the viewing angle dependence of the γ-characteristics by thepixel division method.

According to the second aspect of the present invention, the independentgamma correction is carried out which shifts the chromaticity when thescreen is viewed from the front, from the state maintaining the colorbalance toward blue in the vicinity of the predetermined gradation valuesuch that the gradation dependence of the chromaticity is suppressedwhen viewed from the oblique direction. Such an independent gammacorrection reduces the yellow tinge in the halftone caused by the colorimbalance observed in the oblique view in the conventional color liquidcrystal display device employing the pixel division method, and there isobtained a situation in which the color balance is maintainedsubstantially for the almost whole gradation range in the oblique viewas well as in the front view (to the extent that matters little for ahuman visual sense). As a result, it is possible to display an imagehaving a high color reproducibility when the screen is viewed from anoblique direction as well as when viewed from the front of the screen.

According to the third aspect of the present invention, the independentgamma correction is carried out such that the gradation dependence ofthe chromaticity is suppressed in the oblique view in the vicinity ofthe predetermined gradation value, and also there is obtained asituation in which the color balance in the front view is maintainedsurely for the almost whole gradation values except for the vicinity ofthe predetermined gradation value. Accordingly, it is possible todisplay an image having a sufficiently high color reproducibility forthe almost whole gradation range in the front view, while reducing thecolor imbalance (specifically, the yellow tinge in the halftone)observed in the oblique view in the conventional color liquid crystaldisplay device employing the pixel division method.

According to the fourth aspect of the present invention, the independentgamma correction is carried out such that the gradation dependence ofthe chromaticity is suppressed in the oblique view in the vicinity ofthe predetermined gradation value, and also the independent gammacorrection is carried out such that the curve representing the gradationdependence of the chromaticity in the front view changes approximatelymonotonically with respect to the gradation value. Accordingly, it ispossible to make the chromaticity shift by a change of the gradationvalue not to cause a human sense of discomfort, while reducing the colorimbalance (specifically, the yellow tinge in the halftone) observed inthe oblique view in the conventional color liquid crystal display deviceemploying the pixel division method.

According to the fifth aspect of the present invention, by correctingthe gradation value indicated by the input signal with reference to thecorrection table for the gamma correction associating the gradationvalue before correction and the gradation value after correction witheach other for each of the primary colors for color display, the gammacorrection is carried out in the same manner as in the first aspect ofthe present invention, and thereby it is possible to display an imagehaving a high color reproducibility in the oblique view as well as inthe front view. Also, it is possible to adjust easily a correctionamount in the independent gamma correction, by changing the contents ofthe correction table.

According to the sixth aspect of the present invention, applyingvoltages which are different from each other and change in apredetermined period and in a predetermined amplitude to the first andsecond sub-pixel electrodes provides luminance values different fromeach other to the sub-pixels in each of the pixel formation portions,and also the independent gamma correction is carried out such that thegradation dependence of the chromaticity is suppressed in the obliqueview in the vicinity of the predetermined gradation value determined bythe area ratio of the first sub-pixel electrode to the second sub-pixelelectrode and the difference in the applied voltage between the firstauxiliary electrode and the second auxiliary electrode. Thereby, it ispossible to display an image having a high color reproducibility in theoblique view as well as in the front view, while realizing the pixeldivision method in a comparatively simple configuration to improve theviewing angle dependence of the γcharacteristics.

The seventh to tenth aspects of the present invention have the sameadvantages as those of the first to the fourth aspects of the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a whole configuration of a colorliquid crystal display device according to a first embodiment of thepresent invention.

FIG. 2 is a schematic diagram illustrating a configuration of a displaypart in the first embodiment

FIG. 3 consists of a schematic diagram (A) and an equivalent circuitdiagram (B) respectively illustrating an electrical configuration of apixel formation portion in the display part of the first embodiment.

FIG. 4 consists of signal waveform charts (A to F) for illustratingoperation of the liquid crystal display device according to theembodiment.

FIG. 5 is a characteristic diagram showing VT characteristics (appliedvoltage-transmittance characteristics) in a liquid crystal displaydevice employing the pixel division method.

FIG. 6 is a characteristic diagram showing an example ofγ-characteristics.

FIG. 7 is a characteristic diagram showing different VT characteristicsfor different colors of a pixel in the liquid crystal display device.

FIG. 8 is a VT characteristic diagram for illustrating a problem inassociation with color-reproducibility in the liquid crystal displaydevice employing the pixel division method.

FIG. 9 is a characteristic diagram for illustrating gradation dependenceof chromaticity in the liquid crystal display device employing the pixeldivision method.

FIG. 10 consists of a characteristic diagram (A) and a chromaticitydiagram (B) respectively for illustrating a problem in adjusting a colorbalance to obtain a good color tracking in the liquid crystal displaydevice employing the pixel division method.

FIG. 11 consists of a characteristic diagram (A) and a chromaticitydiagram (B) respectively for illustrating an independent gammacorrection for obtaining the color tracking in the first embodiment.

FIG. 12 is a characteristic diagram showing a state in which a viewingangle dependence of the gamma characteristics changes according to theamplitude of an auxiliary capacitance line voltage in the liquid crystaldisplay device employing the pixel division method.

FIG. 13 is a characteristic diagram showing a state in which the viewingangle dependence of the gamma characteristics changes according to thepixel division ratio in the liquid crystal display device employing thepixel division method.

FIG. 14 is a block diagram showing a configuration of a display controlcircuit in the first embodiment.

FIG. 15 is a diagram for illustrating a correction table for theindependent gamma correction in the first embodiment.

FIG. 16 consists of a characteristic diagram (A) and a chromaticitydiagram (B) respectively for illustrating the independent gammacorrection for obtaining the color tracking in a second embodiment ofthe present invention.

FIG. 17 consists of diagrams (A, B) for illustrating a problem inassociation with the color tracking (gradation dependence of thechromaticity) in the liquid crystal display device employing the pixeldivision method.

DESCRIPTION OF THE REFERENCE SYMBOLS

10 Pixel formation portion

10 a First sub-pixel formation portion

10 b Second sub-pixel formation portion

12 a First TFT (first thin film transistor)

12 b Second TFT (second thin film transistor)

14 a First sub-pixel electrode

14 b Second sub-pixel electrode

16 a First auxiliary electrode

16 b Second auxiliary electrode

20 Gamma correction part

23 Gamma-correction processing part

21 r R gamma-correction table

21 g G gamma-correction table

21 b B gamma-correction table

200 Display control circuit

300 Data-signal-line drive circuit

400 Scanning-signal-line drive circuit

500 Display part

Ccsa First auxiliary capacitance

Ccsb Second auxiliary capacitance

Ecom Common electrode

Vcs1 First auxiliary electrode voltage

Vcs2 Second auxiliary electrode voltage

Vcom Common electrode voltage

Vda First sub-pixel voltage

Vdb Second sub-pixel voltage

CS1 First auxiliary capacitance line

CS2 Second auxiliary capacitance line

G(i) Scanning signal line (i=1 to n)

S(j) Data signal line (j=1 to m)

Vg Gate signal voltage

Vs Data signal voltage

Lr, Lg, Lb Gradation signal (before correction)

Lmr, Lmg, Lmb Gradation signal (after correction)

IL Oblique hue correction range (oblique color imbalance range)

BEST MODES FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described withreference to the accompanying drawings. The following descriptionassumes that a display part employs a vertical alignment mode and isconfigured to provide a normally black display. Here, a drive methodthereof may be a line-inversion drive method in which a voltage appliedto liquid crystal is inverted every frame period and also every one or apredetermined number of scanning signal lines, or a dot-inversion drivemethod in which the voltage applied to the liquid crystal is invertedevery frame period and also every scanning signal line and video signalline.

1. First Embodiment

<1.1 Whole Configuration of a Liquid Crystal Display Device>

FIG. 1 is a block diagram illustrating a whole configuration of anactive-matrix liquid crystal display device according to a firstembodiment of the present invention. This liquid crystal display deviceis provided with: a display control circuit 200; a pixel electrode drivecircuit including a data-signal-line drive circuit (also called “sourcedriver”) 300, a scanning-signal-line drive circuit (also called “gatedriver”) 400, and a common electrode drive circuit (not shown in thedrawing); an auxiliary electrode drive circuit 600; and a display part500. The display part 500 includes a plurality of (m) data signal linesS(1) to S(m), a plurality of (n) scanning signal lines G(1) to G(n), anda plurality of (m×n) pixel formation portions provided correspondinglyto respective intersections of the plurality of data signal lines S(1)to S(m) and the plurality of scanning signal lines G(1) to G(n). Thesepixel formation portions include three kinds of pixel formation portionscorresponding to the three primary colors for displaying a color image,that is, an R pixel formation portion forming an R (red) pixel, a Gpixel formation portion forming a G (green) pixel, and a B pixelformation portion forming a B (blue) pixel. Three pixel formations 10 ofR pixel formation portion, G pixel formation portion and G pixelformation portion, neighboring in a horizontal direction as shown inFIG. 2, constitute one of units of display, which are arranged in amatrix on the display part 500.

The present embodiment employs the pixel division method for improving aviewing angle dependence of display characteristics, and each pixelformation portion 10 in the display part 500 is configured as shown in(A) and (B) of FIG. 3. Here, (A) of FIG. 3 is a schematic diagramshowing an electrical configuration of one pixel formation portion inthe display part 500 and (B) of FIG. 3 is an equivalent circuit diagramshowing the electrical configuration in the pixel formation portion. Asshown in these (A) and (B) of FIG. 3, each pixel formation portion 10includes a first and a second sub-pixel formation portions 10 a and 10 bhaving sub-pixel electrodes 14 a and 14 b independent from each other,respectively, and an average of a luminance value of a sub-pixel formedby the first sub-pixel formation portion 10 a and a luminance value of asub-pixel formed by the second sub-pixel formation portion 10 b becomesa luminance value of a pixel formed by the pixel formation portion 10.

In each of the pixel formation portion 10, the first sub-pixel formationportion 10 a includes a first TFT 12 a, the gate terminal of which isconnected to a scanning signal line G(i) passing through an intersectioncorresponding to the pixel formation portion 10 and the source terminalof which is connected to a data signal line S(j) passing through theintersection, a first sub-pixel electrode 14 a connected to the drainterminal of the first TFT 12 a, and a first auxiliary electrode 16 adisposed so as to form a first auxiliary capacitance Ccsa between thefirst sub-pixel electrode 14 a and the same. Also, the second sub-pixelformation portion 10 b includes a second TFT 12 b, the gate terminal ofwhich is connected to the scanning signal line G(i) passing through theintersection and the source terminal of which is connected to the datasignal line S(j) passing through the intersection, a second sub-pixelelectrode 14 b connected to a drain terminal of the second TFT 12 b, anda second auxiliary electrode 16 b disposed so as to form a secondauxiliary capacitance Ccsb between the second sub-pixel electrode 14 band the same. Further, each of the pixel formation portion 10 includes aliquid crystal layer as an electro-optical element provided commonly atall the pixel formation portions 10 and sandwiched between a commonelectrode Ecom provided commonly at all the pixel formation portions 10and the first and second sub-pixel electrodes. A first liquid crystalcapacitance Clca is formed by the first sub-pixel electrode 14 a, thecommon electrode Ecom and the liquid crystal layer sandwichedtherebetween, and a second liquid crystal capacitance Clcb is formed bythe second sub-pixel electrode 14 b, the common electrode Ecom and theliquid crystal layer sandwiched therebetween. Hereinafter, a sum of thefirst liquid crystal capacitance Clca and the first auxiliarycapacitance Ccsa is referred to as a “first sub-pixel capacitance” anddesignated by a symbol “Cpa”, and a sum of the second liquid crystalcapacitance Clcb and the second auxiliary capacitance Ccsb is referredto as a “second sub-pixel capacitance” and designated by a symbol “Cpb”.Capacitance values of these capacitances Clca, Clcb, Ccsa, Ccsb, Cpa andCpb are also designated by the same symbols Clca, Clcb, Ccsa, Ccsb, Cpaand Cpb.

As shown in (A) and (B) FIG. 3, in the display part 500 there aredisposed a first auxiliary capacitance line CS1 and a second auxiliarycapacitance line CS2 in parallel to the scanning signal line G(i) so asto sandwich each of the pixel formation portion 10 in addition to theforegoing data signal lines S(1) to S(m) and scanning signal lines G(1)to G(n), and the first auxiliary capacitance line CS1 is disposed on oneside of each pixel formation portion 10 (upper side of (A) and (B) ofFIG. 3) and the second auxiliary capacitance line CS2 is disposed on theother side of each pixel formation portion 10 (lower side of (A) and (B)of FIG. 3). The first auxiliary capacitance line CS1 is connected to theauxiliary electrode 16 a of the first sub-pixel formation portion 10 aand the second auxiliary capacitance line CS2 is connected to theauxiliary electrode 16 b of the second sub-pixel formation portion 10 bin each pixel formation portion 10. Accordingly, the first sub-pixelelectrode 14 a is connected to the data signal line S (j) via the firstTFT 12 a and also connected to the first auxiliary capacitance line CS1via the first auxiliary capacitance Ccsa, and the second sub-pixelelectrode 14 b is connected to the data signal line S (j) via the secondTFT 12 b and also connected to the second auxiliary capacitance line CS2via the second auxiliary capacitance Ccsb.

As shown in FIG. 1, the display control circuit 200 receives a datasignal DAT and a timing control signal TS transmitted from outside andoutputs a digital image signal DV, a source start pulse signal SSP, asource clock signal SCK, a latch strobe signal LS, a gate start pulsesignal GSP, a gate clock signal GCK, etc. The digital image signal DV isa signal representing an image to be displayed on the display part 500,and the source start pulse signal SSP, source clock signal SCK, latchstrobe signal LS, gate start pulse signal GSP, gate clock signal GCK,etc. are timing signals for controlling timings for displaying the imageon the display part 500.

The data-signal-line drive circuit 300 receives the digital image signalDV, the source start pulse signal SSP, source clock signal SCK and latchstrobe signal LS outputted from the display control circuit 200, andapplies the data signal to each of the data signal lines S (1) to S (m)for charging the first sub-pixel capacitance Cpa (=Clca+Ccsa) and thesecond sub-pixel capacitance Cpb (=Clcb+Ccsb) in each pixel formationportion 10 of the display part 500. At this time, the data signal-linedrive circuit 300 holds sequentially the digital image signal DVexhibiting voltages to be applied to data signal lines S (1) to S (m),respectively, at each timing when a pulse of the source clock signal SCKis generated. Then, the held digital image signal DV is converted intoanalog voltages at a timing when a pulse of the latch strobe signal LSis generated, and the analog voltages are applied to all the data signallines S (1) to S (m), respectively as data signal voltages, at the sametime.

The scanning-signal-line drive circuit 400 applies active scanningsignals (scanning signal voltages Vg (=VgH) to switch on the first TFT12 a and the second TFT 12 b) sequentially to the scanning signal linesG (1) to G (n) based on the gate start pulse signal GSP and the gateclock signal GCK outputted from the display control circuit 200.

The auxiliary electrode drive circuit 600 generates a first auxiliaryelectrode voltage Vcs1 and a second auxiliary electrode voltage Vcs2based on the timing signal provided from the display control circuit200, and applies these voltages Vcs1 and Vcs2 to the first auxiliarycapacitance line CS1 and the second auxiliary capacitance line CS2 ofthe display part 500, respectively.

The common electrode drive circuit (not shown in the drawing) applies apredetermined voltage to the common electrode Ecom as a common electrodevoltage Vcom. In the present embodiment, the common electrode voltageVcom is assumed to be a fixed voltage.

<1.2 Operation of the Liquid Crystal Display Device>

Operation of the liquid crystal display device configured as describedabove according to the present embodiment will be described withreference to signal waveform charts shown in FIG. 4.

Now attention is focused on the pixel formation portion 10 composed ofthe first sub-pixel formation portion 10 a and the second sub-pixelformation portion 10 b shown in (A) and (B) of FIG. 3. A data signalvoltage Vs as shown in (A) of FIG. 4 is applied to the data signal linecorresponding to this pixel formation portion 10 (hereinbelow, referredto as “corresponding data signal line”) S (j), and the scanning signalvoltage Vg as shown in (B) of FIG. 4 is applied to the scanning signalline corresponding to this pixel formation portion 10 (hereinbelow,referred to as “corresponding scanning signal line”) G (i). Meanwhile,to the first auxiliary capacitance line CS1 is applied a periodicallyvarying voltage having a rectangular waveform with an amplitude of Vcsas shown in (C) of FIG. 4, as a first auxiliary electrode voltage Vcs1,and to the second auxiliary capacitance line CS2 is applied aperiodically varying voltage having a rectangular waveform with theamplitude of Vcs as shown in (D) of FIG. 4 as a second auxiliaryelectrode voltage Vcs2. Here, the first auxiliary electrode voltage Vcs1and the second auxiliary electrode voltage Vcs2 have the same amplitudeand phases different from each other by 180 degrees.

When the data signal voltage Vs, scanning signal voltage Vg, and thefirst and second auxiliary electrode voltages Vcs1 and Vcs2 describedabove are applied by the data-signal-line drive circuit 300,scanning-signal-line drive circuit 400 and auxiliary electrode drivecircuit 600, a voltage of the first sub-pixel electrode 14 a(hereinbelow, referred to as “first sub-pixel voltage”) Vda and avoltage of the second sub-pixel electrode 14 b (hereinbelow, referred toas “second sub-pixel voltage”) Vdb change as follows. That is, when thescanning signal voltage Vg changes from an off voltage VgL to an onvoltage VgH (corresponding scanning signal line G (i) is selected), thefirst TFT T12 a and the second TFT 12 b are changed from an off state toan on state, and the data signal voltage Vs (voltage of positivepolarity with reference to the common electrode voltage Vcom) at thistiming is applied to the first sub-pixel electrode 14 a via the firstTFT 12 a and the second sub-pixel electrode 14 b via the second TFT 12b. Thereby, both of the first and second sub-pixel voltages Vda and Vdbbecome equal to the data signal voltage Vs. Then, when the scanningsignal voltage Vg changes to the off voltage VgL (corresponding scanningsignal line G (i) becomes a non-selected state), both of the first TFT12 a and the second TFT 12 b are changed from the on state to the offstate. At this time, the change of the scanning signal voltage Vg (VgHto VgL) provides an effect to the first and second sub-pixel voltagesVda and Vdb and reduces these voltages Vda and Vdb via parasiticcapacitances Cgd between the gates and the drains of the first andsecond TFTs 12 a and 12 b. This phenomenon is called a “pull-inphenomenon” and a voltage reduction in this case ΔV is called a “pull-involtage” (ΔV>0).

Subsequently, the first auxiliary electrode voltage Vcs1 is increased bythe amplitude of Vcs and the second auxiliary electrode voltage Vcs2 isdecreased by the amplitude of Vcs ((C) and (D) of FIG. 4). Then, thefirst and second auxiliary electrode voltages Vcs1 and Vcs2 repeatalternately the increase and decrease by the amplitude Vcs in thepredetermined period until the scanning signal voltage Vg is changednext to the on voltage VgH (until the corresponding scanning signal lineG (i) is selected). Note that the first and second auxiliary electrodevoltages Vcs1 and Vcs2 have phases different from each other by 180degrees. While the scanning signal line voltage Vg is the off voltage(while the corresponding scanning signal line G (i) is in thenon-selected state and the first and second TFTs 12 a and 12 b are inthe off state), the first sub-pixel voltage Vda is affected by theperiodic change of the first auxiliary electrode voltage Vcs1 via thefirst auxiliary capacitance Ccsa and changes as shown in (E) of FIG. 4,and the second sub-pixel voltage Vdb is affected by the periodic changeof the second auxiliary electrode voltage Vcs2 via the second auxiliarycapacitance Ccsb and changes as shown in (F) of FIG. 4.

When the scanning signal line voltage Vg is changed next to the onvoltage VgH, the data signal voltage Vs (voltage of negative polaritywith reference to the common electrode voltage Vcom) at this timing isapplied to the first sub-pixel electrode 14 a via the first TFT 12 a andthe second sub-pixel electrode 14 b via the second TFT 12 b. Then, whenthe scanning signal voltage Vg is changed to the off voltage VgL, bothof the first TFT 12 a and the second TFT 12 b go into the off state. Atthis time, by the pull-in phenomenon caused by the parasiticcapacitances Cgd between the gates and drains of the first and secondTFTs 12 a and 12 b, the first and second sub-pixel voltages Vda and Vdb,which are voltages of negative polarity, are reduced by about ΔV (ΔV>0).After that, in the same manner as above, the first and second auxiliaryelectrode voltages Vcs1 and Vcs2 repeat alternately the increase anddecrease by the amplitude Vcs in the predetermined period until thescanning signal voltage Vg is changed next to the on voltage VgH.Thereby, the first sub-pixel voltage Vda is affected by the periodicchange of the first auxiliary electrode voltage Vcs1 via the firstauxiliary capacitance Ccsa and changes as shown in (E) of FIG. 4, andthe second sub-pixel voltage Vdb is affected by the periodic change ofthe second auxiliary electrode voltage Vcs2 via the second auxiliarycapacitance Ccsb and changes as shown in (F) of FIG. 4.

Here, when the data signal line voltage Vs is designated by Vsp in thepositive polarity and designated by Vsn in the negative polarity, aneffective value Vlca_rms of an applied voltage to the liquid crystal(hereinbelow referred to as a “first sub-pixel liquid crystal voltage”)in the first sub-pixel formation portion 10 a is provided as followsaccording to (E) of FIG. 4,

Vlca _(—) rms=Vsp−ΔV+(1/2)Vcs(Ccsa/Cpa)−Vcom   (1),

and an effective value Vlcb_rms of an applied voltage to the liquidcrystal (hereinbelow referred to as a “second sub-pixel liquid crystalvoltage”) in the second sub-pixel formation portion 10 b is provided asfollows according to (F) of FIG. 4,

Vlcb _(—) rms=Vsp−ΔV−(1/2)Vcs(Ccsb/Cpb)−Vcom   (2).

From the equations (1) and (2), the effective value of the firstsub-pixel liquid crystal voltage Vlca_rms is larger than that of thesecond sub-pixel liquid crystal voltage Vlcb_rms. Further, when thefirst and second liquid crystal capacitances Clca and Clcb are assumedto be approximately the same as each other and also the first and secondauxiliary capacitances Ccsa and Ccsb are assumed to be the same as eachother (Clca=Clcb and Ccsa=Ccsb), and thereby Cp=Cpa=Cpb is assumed, adifference between the effective value of the first sub-pixel liquidcrystal voltage Vlca_rms and the effective value of the second sub-pixelliquid crystal voltage Vlcb_rms, ΔVlc=Vlca_rms−Vlcb_rms, becomes

ΔVlc=Vcs(Ccs/Cp)   (3).

Therefore, the difference ΔVlc between the effective value of the firstsub-pixel liquid crystal voltage Vlca_rms and the effective value of thesecond sub-pixel liquid crystal voltage Vlcb_rms is proportional to theamplitude of the auxiliary electrode voltage Vcs and can be controlledby this amplitude Vcs.

In the pixel division method described above, the effective value of thefirst sub-pixel liquid crystal voltage Vlca_rms becomes higher and theeffective value of the second sub-pixel liquid crystal voltage Vlcb_rmsbecomes lower than an apparent applied-voltage onto the liquid crystalin the pixel formation portion 10, Vlc_ap=Vsp−ΔV−Vcom. Therefore, arelationship between the apparent applied voltage V=Vlc_ap andtransmittance T (VT characteristics) becomes as shown in FIG. 5. Thatis, the VT characteristics of the first sub-pixel formation portion 10 abecomes as shown by a characteristic curve VTa and the VTcharacteristics of the second sub-pixel formation portion 10 b becomesas shown by a characteristic curve VTb. Further, the VT characteristicsof the pixel formation portion 10 become average characteristicsprovided by these VT characteristics curves VTa and VTb, that is,characteristics as shown by a dotted line in FIG. 5.

In the present embodiment, when voltages according to the data signalDAT (gradation values thereof), which is the input signal from outside,are applied to the liquid crystal in the first and second sub-pixelformation portions 10 a and 10 b, the light transmittance is controlledaccording to the above described VT characteristics in each of the pixelformation portions 10 of the display part 500, and thereby an imageexhibited by the data signal DAT of the input signal is displayed.Further, by employing the pixel division method as described above, theviewing angle dependence of the γ-characteristics is improved in theliquid crystal display device.

<1.3 Color Tracking and Independent Gamma Correction>

A video signal such as a television signal or the like assumes theγ-characteristics of a CRT (Cathode Ray Tube) display device, that is,γ-characteristics as shown in FIG. 6. Accordingly, to reproduce(display) an image having a good gradation from such a video signal inthe liquid crystal display device, a gradation value or the likeindicated by the input signal needs to be corrected according to the VTcharacteristics of the liquid crystal display device (refer to, e.g.,FIG. 5), such that the relationship between the gradation valueindicated by the input video signal and a luminance value of an image tobe displayed, that is, γ-characteristics of the liquid crystal displaydevice becomes the γ-characteristics as shown in FIG. 6. Such gammacorrection methods include a method to correct a gradation valueindicated by the input signal using a lookup table as a correctiontable, and a method to adjust a voltage division ratio in a voltagedivision circuit (gradation voltage generation circuit) for generating agradation voltage to be used in generation of the data signal voltage Vs(refer to, e.g., Japanese Unexamined Patent Application Publication No.2002-258813 (patent reference 1) and Japanese Unexamined PatentApplication Publication No. 2001-222264 (patent reference 2)).

The display part of the color liquid crystal display device includesthree kinds of the pixel formation portions, R, G, and B pixel formationportions, as shown in FIG. 2. Generally, the VT characteristics (appliedvoltage-transmittance characteristics) of the color liquid crystaldisplay device are different slightly among these three kinds of thepixel formation portions as shown in FIG. 7. Therefore, in a casewithout the independent gamma correction, when a video signal exhibitingachromatic gradation values (monochrome signal) is inputted to the colorliquid crystal display device and the gradation value of the monochromesignal is changed, the chromaticity of the displayed image changesconsiderably against the gradation as shown in FIG. 9 (hereinbelow, acurve representing a gradation dependence of the chromaticity of adisplayed image obtained when a video signal exhibiting such achromaticgradation values is inputted is referred to as a “color trackingcurve”). Here, x and y of vertical axes in FIG. 9 are x-y coordinates inthe XYZ colorimetric system introduced by the CIE (CommissionInternationale de l'Eclarirange) (same in FIG. 10, FIG. 11, and FIG. 16to be referred to hereinbelow).

This FIG. 9 shows that the chromaticity changes toward blue as agradation value is reduced from 255 in a displayed image of the colorliquid crystal display device. In this manner, the chromaticity changesconsiderably and satisfactory chromaticity characteristics are notobtained, although a video signal exhibiting achromatic gradation valuesis inputted.

Accordingly the independent gamma correction is carried out such thatthe color balance in the front-view does not change against thegradation, and thereby flat chromaticity characteristics are obtainedagainst the gradation as shown in (A) of FIG. 10. In an example shown in(A) of FIG. 10, gradation values of 0 to 255 are allotted to each R, G,and B (gradation display with eight bits each).

Note that, in the present embodiment, the chromaticity characteristicsare adjusted to become flat in a gradation range of 32 to 255. This isbecause there is a limit to a range where the chromaticity can becorrected by the R, G, and B independent gamma correction of the liquidcrystal while suppressing a black luminance, since the chromaticity in agradation near black is determined by a light leak in polarizer platesin the cross-nicol state and a color filter (CF). Accordingly, in thepresent embodiment, the R, G, and B independent gamma correction iscarried out such that the chromaticity comes gradually close to thechromaticity of black (zero gradation value) in a range below agradation value of 32 as shown in (A) of FIG. 10. Thereby, the colorbalance can be maintained in the gradation value range of 32 to 255,when the screen of the liquid crystal display device is viewed from thefront (in the front view).

In the color liquid crystal display device employing the pixel divisionmethod such as the present embodiment, a luminance value of a pixel (anyof R, G, and B pixels) formed by each pixel formation portion 10 isdetermined by a transmittance value based on the average characteristiccurve (characteristic curve shown by a dotted line in FIG. 8) of thecharacteristic curves VTa and VTb which represent the VT characteristicsof the first and second sub-pixel formation portions 10 a and 10 bcomposing the pixel formation portion 10 as shown in FIG. 8. That is,light flux of each pixel formation portion 10 is a sum of a light fluxof the first sub-pixel formation portion 10 a and a light flux of thesecond sub-pixel formation portion 10 b which are determined by thetransmittance values based on the above described two characteristiccurves VTa and VTb, respectively. Therefore, in the pixel divisionmethod, blue transmittance decreases and the color balance breaks in twovoltage ranges IVa and IVb as shown in FIG. 8, since the bluetransmittance decreases at some gradient values as shown in FIG. 7.

As described hereinabove, the retardation of the liquid crystal has awavelength dependence and thereby VT characteristics are different amongthe three kinds of pixels R, G, and B and these differences are largerwhen a screen is viewed from the oblique direction (in the oblique view)than when viewed from the front (in the front view). Therefore, as shownin (A) of FIG. 10, even when a flat color tracking curve is obtained inthe front view (in the gradation range of 32 to 255), a color imbalancerange IL appears in the halftone in the oblique view. That is, when avideo signal exhibiting the achromatic gradation values is inputted andthe gradation value is changed from 0 to 255, a trajectory on thechromaticity diagram in the front view is obtained as shown by a solidline in (B) of FIG. 10 and a trajectory in the oblique view is obtainedas shown by a dotted line in (B) of FIG. 10. This means that there iscaused the yellow tinge in the color imbalance range IL of the halftonein the oblique view (refer to (B) of FIG. 17).

Accordingly, in the present embodiment, the independent gamma correctionis carried out such that chromaticity coordinate values x and y, whichrepresent a chromaticity in the front view in the color imbalance rangeIL of the halftone, become slightly smaller than those of the statemaintaining the color balance as shown in (A) of FIG. 11, when the videosignal exhibiting the achromatic gradation values is input and thegradation value is changed. Thereby, in a color tracking curve in theoblique view, as shown in (A) of FIG. 11, the chromaticity coordinatevalues x and y are suppressed from increasing from those of the statemaintaining the color balance in the color imbalance range IL. Note thatthe color balance is maintained in the gradation value range of 32 to255 except for the color imbalance range IL. That is, the color trackingcurve representing the gradation dependence of the chromaticity is flatin that range.

In such an independent gamma correction, the color tracking curve in thefront view corresponds to a trajectory on the chromaticity diagram shownby a solid line in (B) of FIG. 11, and the color tracking curve in theoblique view corresponds to a trajectory on the chromaticity diagramshown by a dotted line in (B) of FIG. 11. As apparent from thesetrajectories, in the independent gamma correction of the presentembodiment, the chromaticity in the front view is shifted toward blue inthe color imbalance range IL of the halftone, and thereby the yellowtinge in the oblique view can be reduced. Here, the shift amount towardblue in the front view in the color imbalance range IL (reduced amountsof the chromaticity coordinates x and y) is determined such that theblue tinge does matter little for a human visual sense in the front viewand also the yellow tinge does matter little for the human visual sensein the oblique view (hereinbelow, such color balance adjustment isreferred to as “oblique color imbalance correction”). Note that, here,an angle (acute angle) formed by the normal line of a screen and avisual axis of a screen viewer is referred to as a “viewing angle”. Inthe present embodiment, viewing the screen from an oblique direction of45 degrees (a viewing angle of 45 degrees) is referred to as “theoblique view”, but viewing the screen from an oblique direction ofanother angle, for example, 60 degrees may also be referred to as “theoblique view”.

To carry out the independent gamma correction for obtaining the colortracking curve as described above ((A) of FIG. 11) in the presentembodiment, it is necessary to specify the color imbalance range IL ofthe halftone as a range where the oblique color imbalance correction iscarried out (hereinbelow, this range is referred to as an “oblique huecorrection range). This oblique hue correction range IL appears due toemployment of the, pixel division method, and a position thereof(gradation value breaking the color balance) depends on the pixeldivision ratio and the difference in the effective value of the liquidcrystal voltage ΔVlc=Vcs (Ccs/Cp) between the effective value of thefirst sub-pixel liquid crystal voltage Vlca_rms and the effective valueof the second sub-pixel liquid crystal voltage Vlcb_rms. That is, in thepresent embodiment, the position depends on the area ratio of the firstsub-pixel electrode 14 a to the second sub-pixel electrode 14 b in eachpixel formation portion 10 and the amplitude Vcs of the first and secondauxiliary electrode voltages Vcs1 and Vcs2. Hereinbelow, this point willbe described with reference to FIGS. 12 and 13.

FIG. 12 is a characteristic diagram showing how the viewing angledependence of the γ-characteristics changes according to the amplitudeof the auxiliary capacitance line voltage in the liquid crystal displaydevice employing the pixel division method. Specifically, thischaracteristic diagram shows a relationship between a gradation value inthe front view (hereinbelow, referred to as “front gradation”) and agradation value in the oblique view (hereinbelow, referred to as“oblique gradation”), when an image with the gradation is displayed onthe screen. A curve representing this relationship is referred to as a“viewing angle dependence curve” and the horizontal axis thereofrepresents the front gradation calculated from

255×(front viewing angle normalized transmittance ratio/100)̂(1/2.2)  (4),

and the vertical axis thereof represents the oblique gradationcalculated from

255×(right 45 degree front viewing angle normalized transmittanceratio/100)̂(1/2.2)   (5).

Also, FIG. 12 shows a straight bold line with a gradient of 1 as areference line VAO, and, as the viewing angle dependence curves comecloser to this reference line VAO, a difference between the frontgradation and the oblique gradation becomes smaller and the viewingangle dependence of the γ-characteristics becomes smaller. In a casewhere the display part 500 employs the vertical alignment mode and isconfigured to have the normally black display, the γ-characteristics aredifferent between in the front view and in the oblique view, and animage becomes to show so called “white floating” in the oblique view,which is not observed in the front view. However, by the configurationin which each pixel is composed of a relatively bright sub-pixel and arelatively dark sub-pixel, that is, by employment of the pixel divisionmethod, the “white floating” in the oblique view is reduced and theviewing angle dependence is improved.

FIG. 12 shows four viewing angle dependence curves in a case where thepixel division ratio is 1:1, that is, in a case where an area ratio ofthe first sub-pixel electrode 14 a to the second sub-pixel electrode 14b is 1:1. A solid line shows a viewing angle dependence curve when theamplitude of the first and second auxiliary electrode voltages Vcs1 andVcs2 (hereinbelow, referred to as “CS amplitude”) Vcs is zero volt, adotted line shows a viewing angle dependence curve when the CS amplitudeVcs is 1.5 V, a dashed-dotted line shows a viewing angle dependencecurve when the CS amplitude Vcs is 3.5 V, and a broken line shows aviewing angle dependence curve when the CS amplitude Vcs is 5.5 V.

As shown in FIG. 12, in the viewing angle dependence curve, a bendingcurvature at a bending part (corresponding to an inflection point) shownby a circle becomes larger and the bending part shifts in a directionindicated by an arrow when the CS amplitude Vcs is changed from 1.5 V to5.5 V. In the oblique view, the color balance is broken in such abending part and the yellow tinge is caused ((B) of FIG. 17). That is,the color imbalance range in the oblique view shifts by the CS amplitudeVcs as shown by a shift of the circle in FIG. 12. Accordingly, to obtainthe color tracking curve as shown in FIG. 11, it is necessary to carryout the independent gamma correction according to the CS amplitude Vcs.Here, to carry out the independent gamma correction according to the CSamplitude Vcs using the foregoing equation (3) means to carry out theindependent gamma correction according to a difference between theapplied voltages on the liquid crystal in the first and second sub-pixelformation portions 10 a and 10 b.

FIG. 13 is a characteristic diagram showing how the viewing angledependence of the γ-characteristics is changed by the pixel divisionratio in the liquid crystal display device employing the pixel divisionmethod and shows a relationship between the front gradation value andthe oblique gradation value (viewing angle dependence curve) calculatedin the same manner as in FIG. 12 for different pixel division ratios.That is, FIG. 13 shows three viewing angle dependence curves in a casewhere the CS amplitude Vcs is 3.5 V. A solid line shows the viewingangle dependence curve when the pixel division ratio, more specifically,an area ratio of one sub-pixel electrode whose luminance is higher ofthe first and second sub-pixel electrodes 14 a and 14 b to the othersub-pixel electrode whose luminance is lower (bright sub-pixel area todark sub-pixel area) is 1:1, a dotted line shows a viewing angledependence curve when the ratio of the bright sub-pixel area to the darksub-pixel area is 1:2, and a dashed and dotted line shows a viewingangle dependence curve when a ratio of the bright sub-pixel area to thedark sub-pixel area is 1:3.

As shown in FIG. 13, in the viewing angle dependence curve, a gradationvalue providing the bending part (part corresponding to an inflectionpoint) pointed by an arrow shifts to the lower gradation side when thepixel division ratio (ratio of the bright sub-pixel area to the darksub-pixel area) is changed from 1:1 to 1:3, that is, when a ratio of thedark sub-pixel area is made larger. In the oblique view, the colorbalance is broken in such a bending part and the yellow tinge (FIG. 17)is caused. That is, the color imbalance range in the oblique viewchanges according to the pixel division ratio as shown by the arrows inFIG. 13. Accordingly, to obtain the color tracking curve as shown inFIG. 11, it is necessary to carry out the independent gamma correctionaccording to the pixel division ratio.

As described above, the position (gradation value) where the colorimbalance range appears in the oblique view depends on the pixeldivision ratio and the CS amplitude Vcs. Accordingly, in the presentembodiment, the oblique color imbalance correction is carried out forthe oblique color imbalance range determined by the pixel division ratioand the CS amplitude Vcs so as to obtain the color tracking curve asshown in FIG. 11. For example, in a case of the viewing angle dependencecharacteristics shown in FIG. 13 (CS amplitude Vcs is 3.5 V), theoblique color imbalance correction is carried out for a vicinity of agradation value of 130 when the pixel division ratio is 1:1, for avicinity of a gradation value of 100 when the pixel division ratio is1:2, and for a vicinity of a gradation value of 90 when the pixeldivision ratio is 1:3.

Next, there will be described a configuration for carrying out theindependent gamma correction in the present embodiment to adjust thecolor balance including the oblique color imbalance correction describedabove.

FIG. 14 is a block diagram showing a configuration of the displaycontrol circuit 200 in the present embodiment. This display controlcircuit 200 includes a gamma correction part 20 and a timing controlpart 25. The data signal DAT is provided from outside to the gammacorrection part 20, and the timing control signal TS is provided fromoutside to the timing control part 25.

The timing control part 25 generates the foregoing source start pulsesignal SSP, source clock signal SCK, latch strobe signal LS, gate startpulse signal GSP, gate clock signal GCK, etc based on the timing controlsignal TS.

The gamma correction part 20 includes a gamma correction processing part23, an R correction table 21 r, a G correction table 21 g, and a Bcorrection table 21 b and, with reference to these correction tables 21r, 21 g, and 21 b, corrects a relationship between a gradation valueindicated by the data signal DAT from outside and a luminance value of apixel formed by the pixel formation portion 10 according to thegradation value independently for each of the primary colors (red,green, and blue). That is, the data signal DAT, which is received by thegamma correction part 20, is composed of an R gradation signal Lrexhibiting an R (red) gradation value, a G gradation signal Lgexhibiting a G (green) gradation value, and a B gradation signal Lbexhibiting a B (blue) gradation value in an image to be displayed. Thegamma correction part 20 carries out an independent gamma correction,which is a combination of the conventional correction (FIG. 10) formaintaining the color balance in the almost whole gradation range(gradation value range of 32 to 255 ) and the oblique color imbalancecorrection according to the pixel division ratio and the CS amplitudeVcs, for the R, G, and B gradation signals Lr, Lg, and Lb so as toobtain the color tracking as shown in (A) of FIG. 11.

The R correction table 21 r is a lookup table associating an R gradationvalue before gamma correction with an R gradation value after gammacorrection, the G correction table 21 g is a lookup table associating aG gradation value before gamma correction with a G gradation value aftergamma correction, and B correction table 21 b is a lookup tableassociating a B gradation value before gamma correction with a Bgradation value after gamma correction.

The gamma correction processing part 23 carries out the independentgamma correction as shown in FIG. 15, for example, on the data signalDAT composed of the R gradation signal Lr, G gradation signal Lg, and Bgradation signal Lb, using these R, G, and B correction tables 21 r, 21g, and 21 b, and outputs a digital image signal DV composed of an Rgradation signal Lmr, G gradation signal Lmg, and a B gradation signalLmb after the correction. That is, the gamma correction processing part23 determines an R gradation value after the gamma correction from agradation value before the gamma correction, that is, an R gradationvalue indicated by the R gradation signal Lr from outside, by referringto the R correction table 21 r, and outputs a signal exhibiting the Rgradation value after the gamma correction as the corrected R gradationsignal Lmr. Also, the gamma correction processing part 23 determines a Ggradation value after the gamma correction from a gradation value beforethe gamma correction, that is, a G gradation value indicated by the Ggradation signal Lg from outside, by referring to the G correction table21 g, and outputs a signal exhibiting the G gradation value after thegamma correction as the corrected G gradation signal Lmg. Further, thegamma correction processing part 23 determines a B gradation value afterthe gamma correction from a gradation value before the gamma correction,that is, a B gradation value indicated by the B gradation signal Lb fromoutside, by referring to the B correction table 21 b, and outputs asignal exhibiting the B gradation value after the gamma correction asthe corrected B gradation signal Lmb.

The digital image signal DV composed of the corrected R gradation signalLmr, corrected G gradation signal Lmg, and corrected B gradation signalLmb outputted in this manner is a signal accommodating the colortracking as shown in (A) of FIG. 11 and provided to the data-signal-linedrive circuit 300 as described hereinabove. Thereby, the display part500 displays a color image exhibited by this digital image signal DV.

<1.4 Generation Method of Data for the Correction Table>

As described above, the independent gamma correction is carried out suchthat the color tracking shown (A) of FIG. 11 is obtained, and, thisgamma correction refers to the R, G, and B correction tables 21 r, 21 g,and 21 b. Accordingly, it is necessary to generate data corresponding tosuch an independent gamma correction for the R, G, and B correctiontable 21 r, 21 g, and 21 b. Such data for the R, G, and B correctiontable 21 r, 21 g, and 21 b (hereinbelow, referred to as “correctiondata”) can be generated by the following steps, for example.

-   (1) First, generate the correction data for carrying out the    independent gamma correction so as to suppress the gradation    dependence of the chromaticity when the screen is viewed from the    front, that is, to obtain the color tracking curve in the front view    as shown in (A) of FIG. 10.-   (2) Next, carry out a chromaticity measurement from directions of 45    degrees on the right and left side, while carrying out the    independent gamma correction based on the correction data.-   (3) Adjust the correction data so as to suppress the gradation    dependence of the chromaticity in the oblique view (specifically,    the yellow tinge) in the oblique hue correction range of the    halftone according to a result of the chromaticity measurement. That    is, adjust the correction data so as to shift the chromaticity in    the front view from the state maintaining the color balance toward    blue in the oblique hue correction range in order to reduce the    yellow tinge in the oblique view in the halftone.

The color tracking shown in (A) of FIG. 11 is obtained by theindependent gamma correction based on the correction data after theadjustment generated as described above. Accordingly, this correctiondata after the adjustment may be used for the data of the R, G, and Bcorrection tables 21 r, 21 g, and 21 b. Here, the method for generatingthe correction data described above is an example, and another methodmay be used for generating the correction data if the correction data isgenerated such that color tracking shown in (A) of FIG. 11 is obtained.

<1.5 Advantages>

In the present embodiment as described above, the independent gammacorrection is carried out such that the values of the chromaticitycoordinates, x and y, in the front view is reduced slightly from thevalues in the state maintaining the color balance in the oblique huecorrection range in the halftone (color imbalance range) IL (thechromaticity in the front view is shifted toward blue), as shown in (A)of FIG. 11. Thereby, the values of the chromaticity coordinates, x andy, in the oblique view is suppressed from increasing above the values inthe state maintaining the color balance in the oblique hue correctionrange IL (the shift of the chromaticity in the oblique view towardyellow is reduced). By such oblique color imbalance correction, thecolor imbalance of the halftone observed in the conventional colorliquid crystal display device employing the pixel division method issuppressed to such an extent that matters little for a human visualsense even in the oblique view, and the color balance comes to bemaintained substantially (to such an extent that matters little for ahuman visual sense) for the almost whole gradation range (gradationvalues of 32 to 255 ) in the oblique view as well as in the front view.As a result, it is possible to realize a display having high colorreproducibility when the screen is viewed from the oblique direction aswell as from the front direction, while improving the viewing angledependence of the γ-characteristics by the pixel division method.

2. Second Embodiment

In the first embodiment, the chromaticity in the front view in theoblique hue correction range (color imbalance range) IL of the halftoneis shifted toward blue, and thereby the shift of the chromaticity towardyellow in the oblique view is reduced in the oblique hue correctionrange IL and the color reproducibility in the oblique view is improved.However, as shown in (A) of FIG. 11, when the gradation value is reducedfrom 255 to 0, a shift amount of the chromaticity from the statemaintaining the color balance is switched from increase to decrease at apredetermined gradation value in the halftone. That is, while thechromaticity shifts toward blue (negative direction) along with areduction in the gradation value in the range of the gradation valueslarger than the predetermined gradation value, the chromaticity shiftstoward yellow (positive direction) along with a reduction in thegradation value in the range of gradation values smaller than thepredetermined gradation value. This means that the color tracking curvehas a local minimum point at the predetermined gradation value. Such anextremal point of the color tracking curve in the halftone makes aviewer to feel an unnatural chromaticity change.

Accordingly, in a color liquid crystal display device according to asecond embodiment of the present invention, the independent gammacorrection is carried out so as not to cause such an unnaturalchromaticity change. Hereinbelow, there will be described a liquidcrystal display device according to such present embodiment. Here, aconfiguration of the present embodiment is the same as that of the firstembodiment except for an configuration of R, G, and B gamma correctiontables and part of a configuration in a display part 500 (details to bedescribed below), and therefore the same part or a corresponding part isdesignated by the same reference symbol and detailed description thereofwill be omitted.

The R, G, and B gamma correction tables 21 r, 21 g, and 21 b in thepresent embodiment are determined such that the independent gammacorrection is carried out by a gamma correction processing part 23 forobtaining a color tracking in the front view as shown by curves of abold solid line and dotted line in (A) of FIG. 16 (refer to FIG. 14).Here, curves of a fine solid line and dotted line represent the colortracking in the front view in the foregoing first embodiment in (A) ofFIG. 16 (refer to (A) of FIG. 11).

In the present embodiment, the independent gamma correction is carriedout by the gamma correction processing part 23 as described below withreference to the R, G, and B γ-correction tables 21 r, 21 g, and 21 b.

An oblique hue correction range IL shown in (A) of FIG. 16 is the sameas the oblique hue correction range IL in the first embodiment anddetermined by the pixel division ratio and the CS amplitude Vcs (referto FIG. 12 and FIG. 13). In the present embodiment, the independentgamma correction is carried out such that chromaticity (values of thechromaticity coordinates x and y) in the front view in this oblique huecorrection range IL becomes the same as the extremal value in thechromaticity of the color tracking curve at the gradation value wherethe chromaticity of the color tracking curve in the front view in thefirst embodiment becomes minimum (a gradation value approximately at thecenter of the oblique hue correction range IL) Le, and such that thechromaticity (values of the chromaticity coordinates x, and y) changesmonotonically along with a change of the gradation value L.

For carrying out such independent gamma correction, correction data(data to be set in the R, G, and B correction tables 21 r, 21 g, and 21b ) can be generated as follows, for example. That is, the correctiondata obtained by the foregoing generation method in the first embodiment(correction data corresponding to the color tracking in the firstembodiment shown by the curves of a fine solid line and dotted line in(A) of FIG. 16) may be adjusted such that a color tracking curve in thefront view changes monotonically as shown by the curves of a bold solidline and dotted line. Note that this correction data generation methodis an example and the correction data may be generated by another methodif the correction data is generated such that the color tracking asshown in (A) of FIG. 16 is obtained.

Here, as shown in (A) of FIG. 16, the present embodiment employs a colorfilter or a polarizer plate having a black chromaticity shifted towardblue in a display part 500 so as to change the color tracking curvemonotonically even in a gradation value range of 0 to 32. As shown in(B) of FIG. 16, a position of black (zero gradation value) B2 (0) of thepresent embodiment in the chromaticity diagram is slightly differentfrom a position of black B1 (0) in the first embodiment or aconventional example. Here, a color filter or a polarizer the same asconventional ones may be used replacing such color filter or polarizerplate.

In the present embodiment as described above, the chromaticity in thefront view is shifted toward blue to the same extent as in the firstembodiment in the oblique hue correction range (color imbalance range)IL in the halftone, and thereby the shift toward yellow in the halftoneis suppressed in the oblique view as in the first embodiment and thecolor reproducibility is improved. Further, the color tracking curve inthe front view changes monotonically and thereby the chromaticity shiftby the gradation value becomes not to provide a sense of discomfort to aviewer differently from the case in the first embodiment.

3. Variation

In the first and second embodiments, the independent gamma correctionbased on the correction tables 21 r, 21 g, and 21 b provides anappropriate color tracking and realizes a display having a high colorreproducibility in the oblique view as well as in the front view.However, a method of such independent gamma correction for improving thecolor reproducibility is not limited to a method to correct thegradation signal Lr, Lg, or Lb according to the correction table, butmay be any method to correct a relationship between a gradation valueindicated by the signal inputted in the liquid crystal display device asa signal representing an image to be displayed-and a luminance value ofR, G, or B pixel according to the gradation value. For example, thegamma correction may be carried out by a configuration providing R, G,and B γ-correction correction voltage generating circuits for generatingR, G, and B gradation voltages from R, G, and B reference inputvoltages, respectively, (individual setting of R, G, and B γ-correctioncurve) as described in Japanese Unexamined Patent ApplicationPublication No. 2002-258813 (patent reference 1).

Although the first and second embodiments divide each pixel into twosub-pixels spatially for improving the viewing angle dependence of theγ-characteristics as shown in (A) of FIG. 3, the present invention canbe applied to a case where each pixel is divided into three or moresub-pixels. In this case, two color imbalance ranges appear in thehalftone, but the independent gamma correction may be carried out suchthat the chromaticity in the front view is shifted toward blue in eachof the color imbalance ranges for reducing the yellow tinge in theoblique view. Thereby, it is possible to realize a display having a goodcolor reproducibility in the oblique view as well as in the front view.

Also, although the first and second embodiments employ the spatial pixeldivision method as described hereinabove ((A) of FIG. 3), the sameproblem exists and the present invention can be applied in the case of aconfiguration in which one frame period is divided into a plurality ofsub-frames and an average luminance value in the plurality of sub-framesbecomes a luminance value of each pixel, that is, in a case in which atemporal pixel division method is employed.

INDUSTRIAL APPLICABILITY

The present invention can be applied to a color liquid crystal displaydevice employing the pixel division method in which each pixel of adisplayed image is composed of a predetermined number of two or moresub-pixels obtained by spatial or temporal division of one pixel.

1-10. (canceled)
 11. A color liquid crystal display device configured toemploy a pixel division method in which each pixel of an image displayedin a screen is configured with two or more sub-pixels obtained byspatial division of one pixel in a division ratio, the devicecomprising: a plurality of pixel formation portions providedcorrespondingly to respective pixels of the image, each of the pixelformation portions configured to form a pixel of any of primary colorsfor color display with the two or more sub-pixels; a drive circuitconfigured to provide each of the pixel formation portions with appliedvoltages respectively corresponding to the two or more sub-pixelscomposing the pixel to be formed by that pixel formation portion, basedon a gradation value indicated by an input signal provided from outsideas a video signal representing the image; and a gamma correction partconfigured to correct a relationship between the gradation valueindicated by the input signal and a luminance value of the pixel to beformed by that pixel formation portion according to the gradation valueindependently for each of the primary colors for color display; whereineach of the pixel formation portions forms the respective pixel bydisplaying the two or more sub-pixels with luminance values differentfrom one another based on the applied voltages, and wherein the gammacorrection part corrects the relationship such that gradation dependenceof chromaticity is suppressed when the screen is viewed from a frontthereof, and also corrects the relationship in a vicinity of apredetermined gradation value such that the gradation dependence ofchromaticity is suppressed when the screen is viewed from an obliquedirection, the predetermined gradation value being determined by an arearatio of the two or more sub-pixels in the one pixel and differences inthe applied voltages among the two or more sub-pixels.
 12. The colorliquid crystal display device of claim 11, wherein the gamma correctionpart is configured to correct the chromaticity when viewed from thefront to be shifted from a state maintaining a color balance toward bluein the vicinity of the gradation value such that the gradationdependence of chromaticity is suppressed when viewed from the obliquedirection.
 13. The color liquid crystal display device of claim 11,wherein the gamma correction part is configured to correct therelationship such that a curve representing the gradation dependence ofchromaticity when viewed from the front becomes approximately flat in arange except for the vicinity of the gradation value.
 14. The colorliquid crystal display device of claim 11, wherein the gamma correctionpart is configured to correct the relationship such that a curverepresenting the gradation dependence of chromaticity when viewed fromthe front changes approximately monotonically with respect to thegradation value.
 15. The color liquid crystal display device of claim11, wherein the gamma correction part includes a correction tableassociating a gradation value before correction with a gradation valueafter correction for each of the primary colors for color display inorder to correct the relationship, and outputs the gradation value aftercorrection associated with the gradation value indicated by the inputsignal referring to the correction table, and wherein the drive circuitis further configured to provide each of the pixel formation portionswith the applied voltages based on the gradation value after correction.16. A gamma correction method for correcting a relationship between agradation value indicated by an input signal provided from outside as avideo signal representing an image and a luminance value of a pixelformed according to the gradation value, in a color liquid crystaldisplay device employing a pixel division method in which each pixel ofthe image displayed on a screen is composed of two or more sub-pixelsobtained by spatial division of one pixel in a division ratio, themethod comprising: a correction to correct the relationshipindependently for each of primary colors for color display; wherein inthe correction, the relationship is corrected such that gradationdependence of chromaticity is suppressed when the screen is viewed froma front thereof, and the relationship in a vicinity of a predeterminedgradation value is also corrected such that the gradation dependence ofchromaticity is suppressed when the screen is viewed from an obliquedirection, the predetermined gradation value being determined by an arearatio of the two or more sub-pixels in the one pixel and differences involtages applied to liquid crystal of the color liquid crystal displaydevice among the two or more sub-pixels.
 17. The gamma correction methodof claim 16, wherein in the correction, the chromaticity when viewedfrom the front is corrected to shift from a state maintaining a colorbalance toward blue in the vicinity of the gradation value such that thegradation dependence of chromaticity is suppressed when viewed from theoblique direction.
 18. The gamma correction method of claim 16, whereinin the correction, the relationship is corrected such that a curverepresenting the gradation dependence of chromaticity when viewed fromthe front becomes approximately flat in a range except for the vicinityof the gradation value.
 19. The gamma correction method of claim 16,wherein in the correction, the relationship is corrected such that acurve representing the gradation dependence of chromaticity when viewedfrom the front changes approximately monotonically with respect to thegradation value.